virtio-serial: add the virtio-serial device.

virtio-serial: add the virtio-serial device.

Create virtio-serial which extends virtio-device, so it can be connected
on virtio-bus.

Signed-off-by: KONRAD Frederic <fred.konrad@greensocs.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Tested-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Acked-by: Amit Shah <amit.shah@redhat.com>
Message-id: 1365512016-21944-2-git-send-email-fred.konrad@greensocs.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>